Microfabrication is the process of fabrication of miniature structures of micrometer scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as “semiconductor manufacturing” or “semiconductor device fabrication.” In addition, the fields of microelectromechanical systems (MEMS), microsystems (European terminology), micromachines (Japanese terminology) and subfields, such as microfluidics/lab-on-a-chip, optical MEMS, RF MEMS, PowerMEMS, BioMEMS, and other extensions into nanoscale dimensions (for example NEMS, for nanoelectromechanical systems) have used, adapted, or extended microfabrication methods. Flat-panel displays and solar cells also use similar techniques.
Generally, the process of microfabrication includes precisely controlled steps to form tiny structures with specific shapes or dimensions. The process of forming these tiny structures may include additive steps where materials are deposited or formed and also may include subtractive steps where materials are removed by patterning and etching or other known techniques. The fabrication of such small and varied devices presents numerous challenges in terms of, for example, process variation, quality control, and structure characterization.
One specific example topic area includes characterization. Because the fabricated devices have dimensions on the micrometer scale or less, the device performance or function may change significantly with only small variations in material or geometric properties. Further, because the processes used are applied to such tiny structures, the variation within processes may cause small variations in the material or geometric properties to be common, even for devices fabricated on a same semiconductor wafer with the same device design and, hence, the same sequence of processing steps during fabrication. Thus, for example, the same design may be applied to 100 devices on a single wafer or on different wafers, but each device has a significantly different performance due to process variations. Characterizing the fabricated devices in order to determine the actual performance may then be useful and not always easy.
One approach to characterizing fabricated devices is by using test structures. Test structures are structures fabricated on a wafer with the designed devices that may be tested during or after fabrication in order to determine material and geometric properties of the fabricated designed device.